More specifically, the invention herein is directed to improved methods of and apparatus for enabling such simultaneity, on the same telephone conventional public line, of the exchange of such digital and voice communication--termed "data-under-voice" communication in said co-pending application, where this term is used in a generic sense not limiting the word "under" to be restricted to lower frequency bands, but broadly meaning transmitting the data with the voice by whatever technique. This is as contrasted with prior art use of public telephone lines either for voice or for digital information as described in said copending application. The problem is to enable, within such channel provided by the public switched telephone network, a full multiplex channel for exchange of digital information while simultaneously maintaining a toll-quality audio channel, as little degraded as possible from that provided by the entire original channel. The basic technique employed in accordance with the invention to achieve such "data-under-voice" communication is time compression of the analog voice signal, alternating this compressed signal with a signal encoding the information to be transmitted in the digital channel.